Fifth generation (5G) wireless communication systems are envisaged to expand usage scenarios and applications with respect to current mobile network generations. Ultra-Reliable Low-Latency Communications (URLLC) with strict latency and reliability requirement was agreed as one key scenario for 5G. URLCC demands an ultra-high delivery reliability of 99.999% (five nines) or even higher, within a delivery latency bound as low as half a millisecond. URLLC is of highest importance to enable a range of application including:                Intelligent transportation systems of connected cars;        Monitoring of smart grids with distributed renewable energy sources;        Factory automation with communication among actuators, sensors and controllers;        Remote surgery, remote machine operation, etc.        
Each scenario might require a different set of latency and reliability requirements, e.g., 3-5 ms latency with 1-10{circumflex over ( )}−5 reliability for smart grids, and 1 ms latency with 1-10{circumflex over ( )}−9 reliability for factory automation, etc.
Candidate communication systems to fulfill such requirements and use-cases are, e.g., Long Term Evolution (LTE) and a newly developed radio access called New Radio (NR) by the 3rd Generation Partnership Project (3GPP).
With respect to LTE uplink (UL) transmissions, a wireless device, such as, for example, a user equipment (UE) waits for the next transmission opportunity to send a scheduling request. Afterwards, a network node, such as, for example, an evolved Node B (eNodeB or eNB) allocates a set of resources to the wireless device. As one part of the URLLC requirements, some standardization works have been done in LTE rel-14 and are on-going for LTE rel-15 to reduce latency down-to sub-milliseconds.
In order to remove the latency due to the waiting time of the scheduling request, UL semi-persistent scheduling (SPS) has been standardized in LTE from rel-8 and new features introduced in rel-14, such as skip padding when the buffer is empty and the periodicity is reduced to 1 millisecond. SPS pre-allocates the transmission resources to the wireless device, in anticipation of possible packet transmissions. Consequently, the latency due to scheduling request is reduced to zero. Wireless devices do not use the allocated resource to transmit data if it does not have any packet to transmit.
Another approach to address latency reductions is to reduce the transport time of data and control signaling, by reducing the length of a transmission time interval (TTI), i.e., the smallest scheduling unit. This is called Short TTI (sTTI). The latency is reduced, due to a smaller waiting time for sending scheduling request
As a result of latency reduction, a higher number of transmissions or re-transmissions through Hybrid Automatic Repeat Request (HARQ) can be allocated within the latency bound to boost the reliability to the target number. These (re)transmissions can be allocated in both time and frequency. The pre-allocated resources in SPS (with more than one transmission opportunities to satisfy the reliability) might be wasted if the wireless device does not have anything to send. For sTTI, more resources are needed in the frequency domain due to short duration in time domain, and this quickly leads to a shortage of frequency resources.